[jfx21u] RFR: 8320267: WebView crashes on macOS 11 with WebKit 616.1

[Kevin Rushforth]

Clean backport of fix for macOS 11 WebKit crash to jfx21u.

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Commit messages:
 - Backport 0d3341704c1ced4f61b5f7f7618e23ec182ae379

Changes: https://git.openjdk.org/jfx21u/pull/35/files
 Webrev: https://webrevs.openjdk.org/?repo=jfx21u&pr=35&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8320267
  Stats: 191 lines in 2 files changed: 15 ins; 171 del; 5 mod
  Patch: https://git.openjdk.org/jfx21u/pull/35.diff
  Fetch: git fetch https://git.openjdk.org/jfx21u.git pull/35/head:pull/35

PR: https://git.openjdk.org/jfx21u/pull/35